Tax evasion is a crime that's much more throughly punished than lying to sense agents or misreporting wealth. That's for obvious reasons, because tax evasion actually takes money from the hands of the government, while the later actions (that are not even consistently considered crimes) just hurt some abstract concept of data gathering, that could lead to better policies in the future, or not - nobody can be sure - for reasons that very few people can understand.

Add to that that the few people in power that do understand how data can improve policies normally have an idea what kind of improvement they'll get, and don't like it (they are in power, if they liked it, they'd already do it). While tax evasion is understood by everybody and weaken they power right now, in visible ways.

A tax does it with the force of law using the systems we already have in place. It's the cheapest, easiest, most likely to work way we have of doing this sort of thing.

Why can't we do it with the force of law using the systems we already have in place, without actually charging the tax?

The tax will make sure they list deductions, which are also useful to know.

Also, the transparency is a benefit, but so is the tax revenue. We should at least cover the expense of the program and consider if there are other taxes that should be reduced.